Gender and the Knee Adduction After ACL Reconstruction
The results of this study showed that females who had undergone ACL reconstruction surgery had a 23% greater knee adduction moment about the operated knee during level walking compared with males who had undergone the same ACL reconstruction procedure. Out of the number of gait variables measured, the knee adduction moment was the only one to show a significant gender difference.
The current results can be compared with the only other study to have reported on the knee adduction moment during level walking following ACL reconstruction. Butler et al reported an adduction moment of 0.36 Nm/kg.m for their ACL group, which was significantly greater than a control group which had an adduction moment of 0.30 Nm/kg.m. Both groups comprised mostly of females. This corresponds to the present study in which the adduction moment in our female ACL patients was 0.38 Nm/kg.m.
ACL reconstruction surgery aims to allow the patient to return to sport participation with normal knee function that does not lead to symptomatic or radiographic evidence of OA in later life. A growing number of studies are reporting radiographic evidence of early onset knee OA following both ACL injury and reconstruction. The most recent of these studies showed a high prevalence of radiographic knee OA in a cohort of 181 patients 10–15 years following patellar tendon ACL reconstruction. The incidence was 62% in patients who had an isolated rupture but increased to 80% in patients with a combined (meniscal or collateral ligament injury or chondral damage) injury. Although knee OA has not been shown to be more prevalent in females compared with males following ACL reconstruction surgery, previous reports have shown the prevalence and severity of knee OA of all types to be greater in women compared with men. As such, the higher knee adduction moment seen in females compared with males in the current study may suggest an increased risk for the development of OA in ACL-reconstructed females and this warrants further exploration.
A potential limitation of the present study is that the male and female ACL patients were not compared with an uninjured control group. Therefore, it is possible that the greater adduction moment seen in the female ACL patients was not due to the reconstruction procedure but instead reflects an intrinsic gender difference. This, however, seems unlikely and is not supported by previous research. In a large gait study of 110 healthy men and women, there were no gender differences for a number of gait measures, including the knee adduction moment, during barefoot level walking. Nonetheless, the premise that the knee adduction moment is increased following ACL reconstruction surgery was based largely on the findings of Butler et al and although we refer to the knee adduction moment as being greater in females compared with males, we do not provide evidence in this study that it is greater than 'normal'. In fact, the adduction moment was not significantly greater than the contralateral side. However, this may simply mean that the contralateral side is not the best comparator, as there is some suggestion of bilateral biomechanical changes after ACL injury and reconstruction. That said, a clear difference between the ACL-reconstructed and control groups was seen in the Butler et al study in which the control group was well matched for activity level.
Although increased knee adduction moments have been related to both the increased severity of OA as well as the progression of knee OA in other populations, there is no evidence to suggest that high adduction moments per se cause OA to develop in joints without cartilage damage. As such, it is relevant to note that abnormal biomechanics in other movement planes may equally contribute to the development or progression of knee OA. A number of studies have reported abnormal tibial rotation movements during the stance phase of both walking and running gait as well as during lunging and pivoting in patients who have undergone ACL reconstruction surgery. However, it is currently not clear what amount of rotational change may be clinically important for the development of degenerative changes in the knee joint. It is possible that degenerative changes may develop from the combined changes in a number of biomechanical variables.
A number of potential load-modifying interventions have been put forward to reduce the knee adduction moment during walking. Footwear interventions, such as shoes with a lateral wedge, have been shown to reduce the knee adduction moment in some studies. The use of a brace as well as various walking strategies, such as an increased toe-out angle, increased mediolateral trunk sway and reduced walking speed, have also been shown to be effective. Some of these strategies may have limited clinical use due to the difficulty in sustaining such modifications over time. Recent research which used a real-time active feedback system to encourage participants to shift pressure to the medial side of the foot during walking and resulted in significant reductions in the knee adduction moment is also promising. However, no research has evaluated the effectiveness of any of these interventions in patients with ACL reconstruction.
Strengths of the present study include the close matching of the male and female participants in terms of age, time between injury and surgery, follow-up period and walking speed. However, the males were taller and heavier than the females, so joint moments were normalised for weight and height to account for this. A single surgeon performed the reconstruction procedures and the rehabilitation protocol was also similar for all patients. This, however, means that the population was homogenous and this may limit the ability to generalise the findings to other populations. Although previous studies have shown strong associations between the magnitude of the knee adduction moment and OA incidence and severity, it is also important to note that it was beyond the scope of this study to investigate the actual cartilage contact forces.
In summary, the results of this study showed that a group of females who underwent ACL reconstruction surgery exhibited a significantly larger knee adduction moment about the operated knee during walking than a group of males who had undergone the same reconstructive procedure. It is currently not clear why females have this greater knee adduction moment. Future research should aim to explore the relationship between greater knee adduction moments and the risk for development of OA in this patient group. Long-term studies are also required to determine whether the adduction moment changes as participants continue to engage in strenuous activities and whether load-modifying interventions are warranted.
Discussion
The results of this study showed that females who had undergone ACL reconstruction surgery had a 23% greater knee adduction moment about the operated knee during level walking compared with males who had undergone the same ACL reconstruction procedure. Out of the number of gait variables measured, the knee adduction moment was the only one to show a significant gender difference.
The current results can be compared with the only other study to have reported on the knee adduction moment during level walking following ACL reconstruction. Butler et al reported an adduction moment of 0.36 Nm/kg.m for their ACL group, which was significantly greater than a control group which had an adduction moment of 0.30 Nm/kg.m. Both groups comprised mostly of females. This corresponds to the present study in which the adduction moment in our female ACL patients was 0.38 Nm/kg.m.
ACL reconstruction surgery aims to allow the patient to return to sport participation with normal knee function that does not lead to symptomatic or radiographic evidence of OA in later life. A growing number of studies are reporting radiographic evidence of early onset knee OA following both ACL injury and reconstruction. The most recent of these studies showed a high prevalence of radiographic knee OA in a cohort of 181 patients 10–15 years following patellar tendon ACL reconstruction. The incidence was 62% in patients who had an isolated rupture but increased to 80% in patients with a combined (meniscal or collateral ligament injury or chondral damage) injury. Although knee OA has not been shown to be more prevalent in females compared with males following ACL reconstruction surgery, previous reports have shown the prevalence and severity of knee OA of all types to be greater in women compared with men. As such, the higher knee adduction moment seen in females compared with males in the current study may suggest an increased risk for the development of OA in ACL-reconstructed females and this warrants further exploration.
A potential limitation of the present study is that the male and female ACL patients were not compared with an uninjured control group. Therefore, it is possible that the greater adduction moment seen in the female ACL patients was not due to the reconstruction procedure but instead reflects an intrinsic gender difference. This, however, seems unlikely and is not supported by previous research. In a large gait study of 110 healthy men and women, there were no gender differences for a number of gait measures, including the knee adduction moment, during barefoot level walking. Nonetheless, the premise that the knee adduction moment is increased following ACL reconstruction surgery was based largely on the findings of Butler et al and although we refer to the knee adduction moment as being greater in females compared with males, we do not provide evidence in this study that it is greater than 'normal'. In fact, the adduction moment was not significantly greater than the contralateral side. However, this may simply mean that the contralateral side is not the best comparator, as there is some suggestion of bilateral biomechanical changes after ACL injury and reconstruction. That said, a clear difference between the ACL-reconstructed and control groups was seen in the Butler et al study in which the control group was well matched for activity level.
Although increased knee adduction moments have been related to both the increased severity of OA as well as the progression of knee OA in other populations, there is no evidence to suggest that high adduction moments per se cause OA to develop in joints without cartilage damage. As such, it is relevant to note that abnormal biomechanics in other movement planes may equally contribute to the development or progression of knee OA. A number of studies have reported abnormal tibial rotation movements during the stance phase of both walking and running gait as well as during lunging and pivoting in patients who have undergone ACL reconstruction surgery. However, it is currently not clear what amount of rotational change may be clinically important for the development of degenerative changes in the knee joint. It is possible that degenerative changes may develop from the combined changes in a number of biomechanical variables.
A number of potential load-modifying interventions have been put forward to reduce the knee adduction moment during walking. Footwear interventions, such as shoes with a lateral wedge, have been shown to reduce the knee adduction moment in some studies. The use of a brace as well as various walking strategies, such as an increased toe-out angle, increased mediolateral trunk sway and reduced walking speed, have also been shown to be effective. Some of these strategies may have limited clinical use due to the difficulty in sustaining such modifications over time. Recent research which used a real-time active feedback system to encourage participants to shift pressure to the medial side of the foot during walking and resulted in significant reductions in the knee adduction moment is also promising. However, no research has evaluated the effectiveness of any of these interventions in patients with ACL reconstruction.
Strengths of the present study include the close matching of the male and female participants in terms of age, time between injury and surgery, follow-up period and walking speed. However, the males were taller and heavier than the females, so joint moments were normalised for weight and height to account for this. A single surgeon performed the reconstruction procedures and the rehabilitation protocol was also similar for all patients. This, however, means that the population was homogenous and this may limit the ability to generalise the findings to other populations. Although previous studies have shown strong associations between the magnitude of the knee adduction moment and OA incidence and severity, it is also important to note that it was beyond the scope of this study to investigate the actual cartilage contact forces.
In summary, the results of this study showed that a group of females who underwent ACL reconstruction surgery exhibited a significantly larger knee adduction moment about the operated knee during walking than a group of males who had undergone the same reconstructive procedure. It is currently not clear why females have this greater knee adduction moment. Future research should aim to explore the relationship between greater knee adduction moments and the risk for development of OA in this patient group. Long-term studies are also required to determine whether the adduction moment changes as participants continue to engage in strenuous activities and whether load-modifying interventions are warranted.